Pressure sensitive adhesive sheet

ABSTRACT

A pressure sensitive adhesive sheet comprising a substrate, an intermediate layer superimposed thereon and a pressure sensitive adhesive layer superimposed on the intermediate layer, characterized in that the pressure sensitive adhesive layer exhibiting an elastic modulus at 23° C. ranging from 5.0×10 4  to 1.0×10 7  Pa, the intermediate layer exhibiting an elastic modulus at 23° C. which is not greater than the elastic modulus at 23° C. of the pressure sensitive adhesive layer, or the intermediate layer exhibiting an elastic modulus at 40° C. of less than 1.0×10 6  Pa. At the time of working of the back of an adherend having a surface whose unevenness height differences are large, this pressure sensitive adhesive sheet is preferably stuck to the adherend surface to thereby protect it during the working and to enable performing the grinding at a uniform thickness without the occurrence of dimples.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional of U.S. patent application Ser.No. 09/443,569 filed Nov. 19, 1999, entitled “Pressure SensitiveAdhesive Sheet and Method of Use Thereof”, which is incorporated byreference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a pressure sensitive adhesivesheet. More particularly, the present invention relates to a pressuresensitive adhesive sheet which, at the time of working of the back of anadherend having a surface whose unevenness (irregularity) heightdifferences are large, is preferably stuck to the adherend surface tothereby protect it during the working.

[0004] 2. Description of Related Art

[0005] In the step of grinding the back of a semiconductor wafer, thesurface thereof having electrical circuits formed thereon is protectedby a pressure sensitive adhesive sheet. The height differences ofcustomary circuits attributed to electrode elements have ranged fromabout 5 to 20 μm. For the wafer having these customary circuits formedthereon, the conventional surface protective sheets can fully protectthe circuits without the occurrence of circuit breakage or wafercracking and thus have been satisfactory.

[0006] However, in recent years, the method of mounting IC chips hasbecome diversified. For example, a packaging method has been developedin which an IC chip circuit surface is arranged in a downward direction.In this packaging method, electrode elements protrude from the circuitsurface and the height differences thereof are 30 μm or more,occasionally greater than 100 μm. Protrudent portions formed on thesurface of semiconductor wafers like those mentioned above are referredto in the art as bumps.

[0007] At least two bumps are generally formed per chip. When the numberof bumps is large, the bump pitch (distance between bumps) may behundreds of microns. Depending on bump pitch pattern and chiparrangement, there occur dense portions where the bumps gather andportions where the bump pattern is sparse. In particular, the peripheryof wafers is likely to become sparse in bump portions because of theabsence of chips. With respect to wafers having the pressure sensitiveadhesive sheet stuck thereto, the thicknesses thereof at dense bumpsites and sparse bump sites conspicuously differ. The grinding of suchwafers having conspicuous thickness differences would cause a problembecause the thickness differences lead to a difference in the groundwafer thickness.

[0008] When the back of a wafer carrying such bumps is subjected togrinding while the wafer surface is protected with a conventionalsurface protective sheet, the wafer back is ground deeply in conformitywith the configuration of the bumps which forms recesses (dimples) onthe back of the wafer with the result that the thickness of the waferbecomes nonuniform. Further, cracking has been encountered, which occursfrom dimple portions, resulting in breakage of the wafer.

[0009] A similar cracking problem is encountered with respect to the inkmarking of failure circuits (bad mark) after the inspection of the wafercircuits.

[0010] For semiconductor wafers having large bumps, preventativemeasures have been taken which include, for example, decreasing thehardness of the substrate film of the surface protective sheet orincreasing the thickness of the pressure sensitive adhesive sheet. Thesemeasures, however, have been unsatisfactory, and the above problems havenot yet been resolved.

[0011] The present invention overcomes the problems found in the priorart and provides a pressure sensitive adhesive sheet which, at the timeof working of the back of an adherend having a surface whose unevenheight differences are large, is preferably stuck to the adherendsurface to thereby protect it during the working. In particular, thepresent invention provides a pressure sensitive adhesive sheet which,when the adherend is to be ground to an extremely small thickness,enables performing the grinding at a uniform thickness without theoccurrence of dimples.

SUMMARY OF THE INVENTION

[0012] A first pressure sensitive adhesive sheet of the presentinvention comprises a substrate, an intermediate layer superimposedthereon, and a pressure sensitive adhesive layer superimposed on theintermediate layer, the above pressure sensitive adhesive layerexhibiting an elastic modulus at 23° C. ranging from 5.0×10⁴ to 1.0×10⁷Pa, the above intermediate layer exhibiting an elastic modulus at 23° C.which is not greater than the elastic modulus at 23° C. of the pressuresensitive adhesive layer.

[0013] A second pressure sensitive adhesive sheet according to thepresent invention comprises a substrate, an intermediate layersuperimposed thereon, and a pressure sensitive adhesive layersuperimposed on the intermediate layer, the above intermediate layerexhibiting an elastic modulus at 40° C. of less than 1.0×10⁶ Pa.

[0014] In the present invention, it is preferred that the substrateexhibit a maximum value of tanδ of dynamic viscoelasticity of at least0.5 at a temperature ranging from −5 to 80° C.

[0015] The substrate preferably has a thickness and Young's moduluswhose multiplication product is in the range of 0.5 to 100 kg/cm.

[0016] Moreover, the method of using the pressure sensitive adhesivesheet according to the present invention comprises sticking the pressuresensitive adhesive sheet to a surface of an adherent and working theadherend at its back while protecting the adherend surface by means ofthe pressure sensitive adhesive sheet.

[0017] Thus, the present invention provides a surface protectivepressure sensitive adhesive sheet which desirably follows unevennessesformed at an adherend surface to thereby absorb uneven heightdifferences, so that a smooth back grinding of the adherend can beperformed without being influenced by the surface unevennesses and,hence, without a thickness difference. Further, the present inventionprovides a method of using a pressure sensitive adhesive sheetcharacterized by the use of this surface protective pressure sensitiveadhesive sheet.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in greater detail below.First of all, the first pressure sensitive adhesive sheet according tothe present invention will be illustrated.

[0019] The first pressure sensitive adhesive sheet of the presentinvention comprises a substrate, an intermediate layer, and a pressuresensitive adhesive layer arrange in this ordered array.

[0020] The pressure sensitive adhesive layer can be formed from variousconventional pressure sensitive adhesives. The elastic modulus at 23° C.of the pressure sensitive adhesive layer is in the range of 5.0×10⁴ to1.0×10⁷ Pa, preferably 6.0×10⁴ to 5.0×10⁶ Pa, and still more preferably8.0×10⁴ to 1.0×10⁶ Pa. When the pressure sensitive adhesive layer iscomposed of an energy radiation (energy beam) curable pressure sensitiveadhesive described later, the elastic modulus is that value exhibited bythe pressure sensitive adhesive layer prior to the energy radiationirradiation.

[0021] When the elastic modulus at 23° C. of the pressure sensitiveadhesive layer is lower than 5.0×10⁴ Pa, it is likely that the pressuresensitive adhesive will ooze from the edges of the pressure sensitiveadhesive sheet, or it is likely for the pressure sensitive adhesive tosuffer from shearing deformation caused by the grinding force because ofinsufficient cohesive strength, with the result that the difference ofground wafer thickness is increased. Further, if a shear force appliedto the pressure sensitive adhesive forces the adhesive into bumprecesses, the danger of the pressure sensitive adhesive remaining on thewafer surface is high. On the other hand, when the elastic modulus at23° C. of the pressure sensitive adhesive layer is higher than 1.0×10⁷Pa, the pressure sensitive adhesive layer becomes hard and itscapability for following bump unevennesses is deteriorated.Consequently, problems such as an increase in the difference of groundwafer thickness and an invasion of cooling water for grinding throughgaps between bumps and the pressure sensitive adhesive sheet are likelyto occur.

[0022] The pressure sensitive adhesive, although not limited at all, canbe selected from among, for example, rubber, acrylic, silicone andpolyvinyl ether based pressure sensitive adhesives. Further, use can bemade of energy radiation curable, heat foaming or water swellablepressure sensitive adhesives.

[0023] Among the energy radiation curable (energy ray curable,ultraviolet curable, electron beam curable) pressure sensitiveadhesives, ultraviolet curable pressure sensitive adhesives arepreferred. With respect to the water swellable pressure sensitiveadhesives, for example, those disclosed in Japanese Patent PublicationNos. 5(1993)-77284 and 6(1994)-101455 are preferably used.

[0024] The energy radiation curable pressure sensitive adhesivegenerally contains an acrylic pressure sensitive adhesive and an energyradiation polymerizable compound as main components.

[0025] For example, low-molecular-weight compounds having in themolecule thereof at least two photopolymerizable carbon to carbon doublebonds that can be converted into a three-dimensional network structureby light irradiation as disclosed in Japanese Patent Laid-openPublication Nos. 60(1985)-196,956 and 60(1985)-223,139 are widely usedas the energy radiation polymerizable compounds to be incorporated inthe energy radiation curable pressure sensitive adhesives. Specificexamples thereof include trimethylolpropane triacrylate,tetramethylolmethane tetraacrylate, pentaerythritol triacrylate,pentaerythritol tetraacrylate, dipentaerythritolmonohydroxypentaacrylate, dipentaerythritol hexaacrylate, 1,4-butyleneglycol diacrylate, 1,6-hexanediol diacrylate, polyethylene glycoldiacrylate and commercially available oligoester acrylates.

[0026] Furthermore, in addition to the above acrylate compounds,urethane acrylate oligomers can be used as the energy radiationpolymerizable compounds. Urethane acrylate oligomers can be obtained byreacting a polyester or polyether-type polyol compound with apolyisocyanate compound such as 2,4-tolylene diisocyanate, 2,6-tolylenediisocyanate, 1,3-xylylene diisocyanate, 1,4-xylylene diisocyanate ordiphenylmethane-4,4-diisocyanate to thereby obtain an isocyanateterminated urethane prepolymer and by reacting the obtained isocyanateterminated urethane prepolymer with a hydroxyl group containing acrylateor methacrylate, such as 2-hydroxyethyl acrylate, 2-hydroxyethylmethacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate,polyethylene glycol acrylate or polyethylene glycol methacrylate.

[0027] With respect to the blending ratio of energy radiationpolymerizable compound to acrylic pressure sensitive adhesive in theenergy radiation curable pressure sensitive adhesive, it is preferredthat 50 to 200 parts by weight, more preferably 50 to 150 parts byweight, and still more preferably 70 to 120 parts by weight of theenergy radiation polymerizable compound be used per 100 parts by weightof the acrylic pressure sensitive adhesive. In this instance, theinitial adhesive strength of the obtained pressure sensitive adhesivesheet is large, and the adhesive strength is sharply decreased uponirradiation of the pressure sensitive adhesive layer with energyradiation. Accordingly, the peeling at the interface of the wafer andthe energy radiation curable pressure sensitive adhesive layer to beperformed after the completion of wafer back grinding is facilitated.

[0028] The energy radiation curable pressure sensitive adhesive may becomposed of an energy radiation curable copolymer having an energyradiation polymerizable group as a side chain. This energy radiationcurable copolymer simultaneously exhibits satisfactory adherence andenergy radiation curability properties. Details of the energy radiationcurable copolymer having an energy radiation polymerizable group as aside chain are described in, for example, Japanese Patent Laid-openPublication Nos. 5(1993)-32,946 and 8(1996)-27,239.

[0029] The time spent for photopolymerization and the photoirradiationdose can be reduced by mixing a photopolymerization initiator into theenergy radiation curable pressure sensitive adhesive.

[0030] This photopolymerization initiator can be a photoinitiator suchas a benzoin compound, an acetophenone compound, an acylphosphine oxidecompound, a titanocene compound, a thioxanthone compound or a peroxidecompound, or a photosensitizer such as an amine or a quinone. Specificexamples thereof include 1-hydroxycyclohexyl phenyl ketone, benzoin,benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether,benzyl diphenyl sulfide, tetramethylthiuram monosulfide,azobisisobutyronitrile, dibenzyl, diacetyl and β-chloroanthraquinone.

[0031] The photopolymerization initiator is preferably added in anamount of 0.05 to 15 parts by weight, still preferably 0.1 to 10 partsby weight, and optimally 0.5 to 5 parts by weight, to 100 parts byweight of the resin total.

[0032] The above acrylic energy radiation curable pressure sensitiveadhesive possesses satisfactory adhesive strength to the wafer beforeirradiation with energy radiation, and the adhesive strength isextremely decreased upon irradiation with energy radiation. That is, theacrylic energy radiation curable pressure sensitive adhesive enablesbonding the wafer with the pressure sensitive adhesive sheet withsatisfactory adhesive strength to thereby attain a surface protectionbefore irradiation with energy radiation, but after irradiation withenergy radiation, enables easily stripping of the sheet from the groundwafer.

[0033] The elastic modulus at 23° C. of the intermediate layer is notgreater than the elastic modulus at 23° C. of the pressure sensitiveadhesive layer. Preferably, the elastic modulus at 23° C. of theintermediate layer is in the range of 1 to 100%, more preferably 10 to90%, and still more preferably 30 to 80% of the elastic modulus at 23°C. of the pressure sensitive adhesive layer.

[0034] When the elastic moduli of the pressure sensitive adhesive layerand the intermediate layer at 23° C. satisfy the above relationship, notonly can the sticking of the pressure sensitive adhesive sheet beeffected in such a manner that the pressure sensitive adhesive sheetsatisfactorily follows bump unevennesses, but also the shear force tothe pressure sensitive adhesive layer is dispersed to thereby preventthe leaving behind of the pressure sensitive adhesive at the time ofstripping. In other words, when the pressure sensitive adhesive sheet isstripped, the pressure sensitive adhesive is stripped along with thepressure sensitive sheet. Furthermore, the sticking can be effected sothat there is no thickness difference between dense bump portions andsparse bump portions on the wafer surface.

[0035] The material composing the intermediate layer is not particularlylimited as long as the above properties can be realized. For example,the material of the intermediate layer can be selected from amongvarious pressure sensitive adhesive compositions such as those based onacrylics, rubbers and silicones, and thermoplastic elastomers andultraviolet curable resins that can be employed in the preparation of asubstrate as described later.

[0036] The intermediate layer is preferably composed of a material whichexhibits a maximum value of tanδ of dynamic viscoelasticity (hereinafterreferred to simply as “tanδ value”) of at least 0.3, more preferably 0.4to 2.0, and still more preferably 0.5 to 1.2, at a temperature rangingfrom 0 to 60° C. The term “tanδ” used herein represents the loss tangentwhich defines the ratio of loss elastic modulus to storage elasticmodulus. For example, it is measured by the use of a dynamicviscoelasticity measuring instrument on the basis of response to astress such as tension or torsion applied to an object.

[0037] The upper surface, i.e., the side on which the pressure sensitiveadhesive layer is formed, of the intermediate layer may be provided withcorona treatment or furnished with another layer such as a primer inorder to increase the adherence to the pressure sensitive adhesive.

[0038] Although various films commonly used in pressure sensitiveadhesive sheets can be employed without any particular limitation as thesubstrate, preferred use is made of films which exhibit a dynamicviscoelasticity “tanδ value” of at least 0.5, especially 0.5 to 2.0, andstill more especially 0.7 to 1.8, at a temperature ranging from −5 to80° C.

[0039] It is preferred that the multiplication product of the thicknessand Young's modulus of the substrate be in the range of 0.5 to 100kg/cm, especially 1.0 to 50 kg/cm, and still more especially 2.0 to 40kg/cm.

[0040] When the multiplication product of the thickness and Young'smodulus of the substrate falls within the above range, the mechanicalaptitude, for example, sticking aptitude of the pressure sensitiveadhesive sheet, is enhanced to thereby increase the work efficiency.

[0041] The substrate is preferably composed of a resin film. Thesubstrate may be prepared by either molding a curable resin into a filmand curing the film or by molding a thermoplastic resin.

[0042] For example, an energy radiation curable resin or a thermosettingresin is used as the curable resin. Preferably, an energy radiationcurable resin is used.

[0043] The energy radiation curable resin is preferably selected fromamong, for example, resin compositions whose principal component is aphotopolymerizable urethane acrylate oligomer and polyene/thiol resins.

[0044] This urethane acrylate oligomer can be obtained by reacting apolyester or polyether type polyol compound with a polyisocyanatecompound such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate,1,3-xylylene diisocyanate, 1,4-xylylene diisocyanate ordiphenylmethane-4,4′-diisocyanate to thereby obtain anisocyanate-terminal urethane prepolymer and by reacting thisisocyanate-terminal urethane prepolymer with a hydroxyl group containingacrylate or methacrylate such as 2-hydroxyethyl acrylate, 2-hydroxyethylmethacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate,polyethylene glycol acrylate or polyethylene glycol methacrylate. Thisurethane acrylate oligomer has a photopolymerizable double bond in itsmolecule and undergoes polymerization and curing when irradiated withlight to thereby form a film.

[0045] The molecular weight of urethane acrylate oligomers preferablyused in the present invention is in the range of 1000 to 50,000,especially 2000 to 30,000. These urethane acrylate oligomers can be usedeither individually or in combination.

[0046] It is often difficult to obtain a film only from the aboveurethane acrylate oligomers. Thus, films are generally obtained bydiluting the urethane acrylate oligomer with a photopolymerizablemonomer, conducting a film formation step and curing the film. Thephotopolymerizable monomer has a photopolymerizable double bond in itsmolecule and, in the present invention, it is preferred to use anacrylic ester compound having a relatively bulky group.

[0047] The photopolymerizable monomer employed for diluting the urethaneacrylate oligomer is, for example, selected from among:

[0048] alicyclic compounds such as isobomyl (meth)acrylate,dicyclopentenyl (meth)acrylate, dicyclopentenyl (meth)acrylate,dicyclopentenyloxy (meth)acrylate, cyclohexyl (meth)acrylate andadamantane (meth)acrylate;

[0049] aromatic compounds such as phenylhydroxypropyl acrylate, benzylacrylate and phenol ethylene oxide modified acrylate; and

[0050] heterocyclic compounds such as tetrahydrofurfuryl (meth)acrylate,morpholinyl acrylate, N-vinylpyrrolidone and N-vinylcaprolactam.According to necessity, use may be made of polyfunctional (meth)acrylates.

[0051] The above photopolymerizable monomer is preferably added in anamount of 5 to 900 parts by weight, more preferably 10 to 500 parts byweight, and optimally 30 to 200 parts by weight, to 100 parts by weightof the urethane acrylate oligomer.

[0052] On the other hand, the photopolymerizable polyene/thiol resin foruse in the production of the substrate is comprised of a polyenecompound having no acryloyl group and a polythiol compound.Specifically, the polyene compound is selected from among, for example,diacrolein pentaerythritol, trimethylolpropane diallyl ether adduct oftolylene diisocyanate and unsaturated allylurethane oligomer.Mercaptoacetic acid or mercaptopropionic acid ester of pentaerythritolcan preferably be used as the polythiol compound. Furthermore, use canbe made of commercially available polyene polythiol oligomers. Themolecular weight of the polyene/thiol resin for use in the presentinvention is preferably in the range of 3000 to 50,000, more preferably5000 to 30,000.

[0053] In the forming of the substrate from the energy radiation curableresin, the time spent for photopolymerization and the photoirradiationdose can be reduced by mixing a photopolymerization initiator into theresin.

[0054] This photopolymerization initiator can be as aforementioned andis preferably added in an amount of 0.05 to 15 parts by weight, morepreferably 0.1 to 10 parts by weight, and optimally 0.5 to 5 parts byweight, to 100 parts by weight of the resin total.

[0055] In the production of the above curable resin, oligomer andmonomer can be selected from among various combinations so that theabove properties can be realized.

[0056] The thermoplastic resin for preparing the substrate can beselected from among, for example, polyolefin resins such aspolyethylene, polypropylene, polybutene, polybutadiene andpolymethylpentene; and nonhydrogenated or hydrogenatedstyrene-vinylisoprene block copolymer. Of these, nonhydrogenated orhydrogenated styrene-vinylisoprene block copolymer is preferred.

[0057] The styrene-vinylisoprene block copolymer is generally a highvinyl bond SIS (styrene-isoprene-styrene block copolymer). Both thenonhydrogenated and hydrogenated styrene-vinylisoprene block copolymersper se exhibit a high peak of tanδ at about room temperature.

[0058] Additives capable of enhancing the tanδ value are preferablyadded to the above resin. Examples of the additives capable of enhancingthe tanδ value include inorganic fillers such as calcium carbonate,silica and mica and metal fillers such as iron and lead. The addition ofmetal fillers having a high specific gravity is especially effective.

[0059] Further, the substrate may contain other additives, for example,colorants such as pigments and dyes in addition to the above components.

[0060] With respect to the film forming method, for example, thesubstrate can be produced by first casting a liquid resin (for example,precure resin or resin solution) onto a processing sheet in the form ofa thin coating and subsequently conducting given steps for filmformation. In this method, the stress loaded on the resin during thefilm formation is small, so that the occurrence of fish eye is reduced.Moreover, the uniformity of the film thickness is high and the thicknessprecision generally falls within 2%.

[0061] Another preferred method of forming a film comprises calenderingor extrusion forming with the use of T die or inflation.

[0062] The pressure sensitive adhesive sheet of the present invention isproduced by forming the pressure sensitive adhesive layer on theintermediate layer superimposed on the above substrate. When thepressure sensitive adhesive layer is composed of an ultraviolet curablepressure sensitive adhesive, the substrate and the intermediate layermust be transparent.

[0063] In the pressure sensitive adhesive sheet of the presentinvention, the thickness of the substrate is preferably in the range of30 to 1000 μm, more preferably 50 to 800 μm, and optimally 80 to 500 μm.

[0064] The thickness of the intermediate layer is preferably in therange of 5 to 100 μm, more preferably 10 to 80 μm, and optimally 20 to60 μm. Further, the thickness of the pressure sensitive adhesive layer,although depending on the type of material composing the same, isgenerally in the range of about 5 to 100 μm, preferably about 10 to 80μm, and optimally about 20 to 60 μm.

[0065] The total thickness of the intermediate layer and the pressuresensitive adhesive layer is appropriately determined taking into accountthe bump height, bump configuration, bump interval pitch, etc. on anadherend to which the pressure sensitive adhesive sheet is stuck. It isgenerally preferred that the total thickness of the intermediate layerand the pressure sensitive adhesive layer be determined so as to be atleast 50%, preferably 60 to 100%, of the bump height. When the totalthickness of the intermediate layer and the pressure sensitive adhesivelayer is so determined, the pressure sensitive adhesive sheet followscircuit surface unevennesses to thereby compensate for and cancel theheight differences.

[0066] The pressure sensitive adhesive sheet of the present inventioncan be obtained by applying a resin which defines an intermediate layeronto a substrate; drying or curing the resin by appropriate means tothereby form the intermediate layer; coating the intermediate layer withthe above pressure sensitive adhesive in appropriate thickness accordingto the customary technique employing a roll coater, a knife coater, agravure coater, a die coater, a reverse coater or the like; and dryingthe pressure sensitive adhesive to thereby form a pressure sensitiveadhesive layer on the intermediate layer. When considered necessary, arelease liner is applied onto the pressure sensitive adhesive layer.

[0067] The second pressure sensitive adhesive sheet according to thepresent invention will now be described.

[0068] The second pressure sensitive adhesive sheet of the presentinvention comprises a substrate, an intermediate layer and a pressuresensitive adhesive layer arranged in this ordered array. Though varioustypes of substrates and pressure sensitive adhesives can be used in thesecond pressure sensitive adhesive sheet, those embodiments explainedfor the first pressure sensitive adhesive sheet are preferably used.

[0069] The intermediate layer of the second pressure sensitive adhesivesheet exhibits an elastic modulus at 40° C. of less than 1.0×10⁶ Pa,preferably 5.0×10³ to 5.0×10⁵ Pa, and more preferably 1.0×10⁴ to 1.0×10⁵Pa.

[0070] When grinding the back of the semiconductor wafer, thetemperature of the pressure sensitive adhesive sheet would be about 40°C. due to grinding heat. Therefore, the elastic modulus of theintermediate layer at this temperature is significant. That is, when theelastic modulus of the intermediate layer at 40° C. is within the aboverange, the pressure sensitive adhesive sheet precisely follows theunevenness of an adherend surface to compensate for the uneven heightdifferences at the time of grinding the back of the adherend, to therebyachieve a smooth back grinding, free of thick difference without beingaffected by the unevenness of the adherend surface regardless of typesof the substrate and pressure sensitive adhesive.

[0071] Preferred embodiments such as thickness, etc. of substrate,intermediate layer and pressure sensitive layer of the second pressuresensitive adhesive sheet are the same as those described above inconnection with the first pressure sensitive adhesive sheet, and theproduction thereof is performed in the same manner as described withrespect to the first pressure sensitive adhesive sheet.

[0072] The first and second pressure sensitive adhesive sheets of thepresent invention are used to protect the surface of various items andto effect temporary fixing thereof at the time of precision working. Inparticular, the pressure sensitive adhesive sheet is suitably used forprotecting the circuit surface during the grinding of the back ofsemiconductor wafers. The pressure sensitive adhesive sheet of thepresent invention has the above-specified structure and henceeffectively absorbs and compensates for the unevennesses of the circuitsurface. Therefore, the pressure sensitive adhesive sheet of the presentinvention can not only be stuck with satisfactory adherence to wafershaving large surface height differences because of bumps and the likebut also mitigates against the influence of the unevennesses of thewafer surface to the back grinding to thereby enable accomplishing anextremely smooth grinding as well as prevention of the wafer breakage.Moreover, when the pressure sensitive adhesive layer is composed of, forexample, an ultraviolet curable pressure sensitive adhesive, theadhesive strength thereof can be easily reduced by ultravioletirradiation so that, after the completion of required working, thepressure sensitive adhesive layer can be easily stripped off byirradiating the pressure sensitive adhesive layer with ultraviolet rays.

[0073] Thus, the present invention provides a surface protectivepressure sensitive adhesive sheet which can precisely follow theunevennesses of an adherend surface to thereby absorb and compensate forthe height differences and which, at the time of grinding of the back ofthe adherend, enables achievement of a smooth back grinding free ofthickness differences without being affected by the unevennesses of theadherend surface.

EXAMPLES

[0074] The present invention will further be illustrated below withreference to the following Examples which in no way limit the scope ofthe invention.

[0075] In the following Examples and Comparative Examples, the “backgrinding aptitude test” was carried out in the following manner.

[0076] Back Grinding Aptitude Test

[0077] The following printed dotted bad marks as bumps were provided ona 6-inch mirror wafer. Each pressure sensitive adhesive sheet was stuckto the wafer surface provided with the bad mark, and the oppositesurface of the wafer was ground. The wafer configuration, grindingconditions and evaluation method were as follows:

[0078] (1) Wafer configuration:

[0079] wafer diameter: 6 inches,

[0080] thickness of wafer (thickness of portion where no dot printingwas made): 650 to 700 μm,

[0081] dot diameter: 500 to 600 μm,

[0082] dot height: 105 μm, and

[0083] dot pitch: 2.0 mm interval (no printing was made up to 20 mm fromthe wafer circumference);

[0084] (2) Back grinding conditions:

[0085] finish thickness: 200 μm, and

[0086] grinding apparatus: grinder DFG 840 manufactured by DiscoCorporation; and

[0087] (3) Evaluation method:

[0088] (3.1) Dimples:

[0089] Ground wafer back was observed. When there was no crack ordimple, the wafer was evaluated as “excellent”. When dimples wereobserved but the maximum depth thereof was less than 2 μm, the wafer wasevaluated as “good”. On the other hand, when dimples were observed andthe maximum depth thereof was 2 μm or more, the wafer was evaluated as“failure”.

[0090] (3.2) Difference of wafer thickness:

[0091] After the completion of back grinding, the pressure sensitiveadhesive sheet was stripped from the wafer. The thickness was measuredat 30 points, including those lying within 20 mm from the wafercircumference, of the wafer. The difference means the wafer thicknessmaximum minus the wafer thickness minimum.

[0092] The measuring was performed by the use of Dial Thickness Gauge(manufactured by Ozaki Mfg. Co., Ltd.).

[0093] tanδ

[0094] The tanδ was measured by a dynamic viscoelasticity measuringinstrument at a 110 Hz tensile stress. Specifically, each substrate wassampled into a given size, and the tanδ was measured at a frequency of110 Hz and at temperatures ranging from −40 to 150° C. with the use ofRheovibron DDV-II-EP manufactured by Orientec Corporation. With respectto the substrate, the maximum value of measured tanδ at temperaturesranging from −5 to 80° C. was defined as “tanδ value”. With respect tothe intermediate layer, the maximum value of measured tanδ attemperatures ranging from 0 to 60° C. was defined as “tanδ value”.

[0095] Young's Modulus

[0096] The Young's modulus was measured at a test speed of 200 mm/min inaccordance with Japanese Industrial Standard (JIS) K7127.

[0097] Elastic Modulus

[0098] The elastic modulus G′ of each of the pressure sensitive adhesiveand the intermediate layer was measured by the torsional shear method,wherein,

[0099] specimen: cylinder of 8 mm diameter×3 mm height,

[0100] instrument: Dynamic Analyzer RDA II (manufactured by Reometric),and

[0101] frequency: 1 Hz.

Example 1

[0102] 50 parts by weight of urethane acrylate oligomer having a weightaverage molecular weight of 5000 (produced by Arakawa Chemical IndustryCo., Ltd.), 25 parts by weight of isobomyl acrylate, 25 parts by weightof phenylhydroxypropyl acrylate, 2.0 parts by weight of1-hydroxycyclohexyl phenyl ketone as a photopolymerization initiator(Irgacure 184 produced by Ciba-Geigy) and 0.2 part by weight ofphthalocyanine pigment were blended together, thereby obtaining anenergy radiation curable resin composition.

[0103] PET film (produced by Toray Industries, Inc., thickness: 38 μm)as a casting processing sheet was coated with the thus-obtained resincomposition at a thickness of 110 μm according to the fountain dietechnique, thereby forming a resin composition layer. Immediately afterthe coating, the resin composition layer was laminated with the same PETfilm and thereafter irradiated with ultraviolet rays emitted from a highpressure mercury lamp (160 W/cm, height 10 cm) at a dose of 250 mJ/cm²so that the resin composition layer was crosslinked and cured. Thus, asubstrate film of 110 μm thickness was obtained. The tanδ and Young'smodulus of this substrate film were measured by the above methods. Theresults are given in Table 1.

[0104] A composition was prepared by blending together 60 parts byweight of urethane acrylate (produced by Toagosei Chemical Industry Co.,Ltd.), 20 parts by weight of phenol ethylene oxide modified acrylate(trade name: M-101, produced by Toagosei Chemical Industry Co., Ltd.),10 parts by weight of isobomyl acrylate and 2.0 parts by weight ofphotopolymerization initiator (Irgacure 184 produced by Ciba-Geigy), andcast on one side of the above substrate film according to the fountaindie technique. Thus, an intermediate layer of 40 μm thickness wasformed. The tanδ and Young's modulus of this intermediate layer weremeasured by the above methods. The results are given in Table 1.

[0105] A pressure sensitive adhesive composition was prepared byblending together 100 parts by weight of acrylic pressure sensitiveadhesive (copolymer of n-butyl acrylate and acrylic acid), 120 parts byweight of urethane acrylate oligomer whose molecular weight was 8000, 10parts by weight of curing agent (diisocyanate compound) and 5 parts byweight of photopolymerization initiator (benzophenone compound), appliedonto the intermediate layer and dried so that a pressure sensitiveadhesive layer of 20 μm thickness was formed. Thus, a pressure sensitiveadhesive sheet was obtained.

[0106] The back grinding aptitude test of the obtained pressuresensitive adhesive sheet was conducted. The results are given in Table1.

Example 2

[0107] The same procedure as in Example 1 was repeated except that a 20μm thick intermediate layer was formed from a composition comprising 100parts by weight of a copolymer of n-butyl acrylate and acrylic acid and5 parts by weight of curing agent (diisocyanate compound) and that thethickness of the pressure sensitive adhesive layer was 40 μm. Theresults are given in Table 1.

Example 3

[0108] The same procedure as in Example 1 was repeated except that thesubstrate was formed by employing 50 parts by weight of isobornylacrylate without using the phenylhydroxypropyl acrylate, that a 20 μmthick intermediate layer was formed from a composition comprising amixture of 100 parts by weight of a copolymer of 2-ethylhexyl acrylate,vinyl acetate and acrylic acid and 5 parts by weight of epoxy curingagent (Tetrad C) and that a 40 μm pressure sensitive adhesive layer wasformed from a composition comprising the acrylic pressure sensitiveadhesive of Example 1 and 5 parts by weight of curing agent(diisocyanate compound). The results are given in Table 1.

Example 4

[0109] The same procedure as in Example 2 was repeated except that 110μm thick low-density polyethylene film (trade name: Sumikathene L705)was used as a substrate film. The results are given in Table 1.

Comparative Example 1

[0110] The same procedure as in Example 1 was repeated except that nointermediate layer was formed. The results are given in Table 1.

Comparative Example 2

[0111] The same procedure as in Example 1 was repeated except that a 40μm thick intermediate layer was formed by applying a compositioncomprising a mixture of 5 parts by weight of curing agent isocyanate and100 parts by weight of an acrylic copolymer of 90 parts by weight ofvinyl acetate, 8 parts by weight of methyl methacrylate and 2 parts byweight of acrylic acid, followed by drying. The results are given inTable 1.

Comparative Example 3

[0112] The same procedure as in Example 4 was repeated except that nointermediate layer was formed and that the thickness of the pressuresensitive adhesive layer was 20 μm. The results are given in Table 1.

Comparative Example 4

[0113] The same procedure as in Example 1 was repeated except that a 120μm thick ethylene/vinyl acetate copolymer film (vinyl acetate content:12%) was used as a substrate film, no intermediate layer was formed anda 10 μm thick pressure sensitive adhesive layer was formed from the samepressure sensitive adhesive as used in Example 1. The results are givenin Table 1.

Comparative Example 5

[0114] The same procedure as in Example 1 was repeated except that a 100μm thick polyethylene terephthalate film was used as a substrate film,no intermediate layer was formed and a 10 μm thick pressure sensitiveadhesive layer was formed from the same pressure sensitive adhesive asused in Example 1. The results are given in Table 1. TABLE 1 SubstratePressure Sensitive Adhesive Back Polish- thickness × Intermediate Layerelastic ing Aptitude thick- Young's thick- elastic elastic moduluselastic difference of ness modulus ness modulus modulus thickness (Pa)modulus (Pa) wafer tanδ (μm) (kg/cm) tanδ (μm) (Pa) 23° C. (Pa) 40° C.(μm) 23° C. 40° C. dimple thickness (μm) Example 1 1.20 110 22.0 1.20 405.5 × 10⁴ 1.8 × 10⁴ 20 1.5 × 10⁵ 5.8 × 10⁴ excellent 3.2 Example 2 1.20110 22.0 1.50 20 1.0 × 10⁵ 3.2 × 10⁵ 40 1.5 × 10⁵ 5.8 × 10⁴ excellent4.8 Example 3 0.75 110 9.0 0.45 20 9.0 × 10⁴ 5.0 × 10⁴ 40 1.0 × 10⁵ 4.1× 10⁴ excellent 6.0 Example 4 0.19 110 14.3 1.50 20 1.0 × 10⁵ 3.2 × 10⁵40 1.5 × 10⁵ 5.8 × 10⁴ good 6.8 Comp. 1.20 110 22.0 20 1.5 × 10⁵excellent 59 Ex. 1 Comp. 1.20 110 22.0 0.5 40 3.0 × 10⁶ 1.7 × 10⁶ 20 1.5× 10⁵ 5.8 × 10⁴ excellent 37 Ex. 2 Comp. 0.19 110 14.3 20 1.5 × 10⁵failure 72 Ex. 3 Comp. 0.17 120 6.6 10 1.5 × 10⁵ failure 66 Ex. 4 Comp.0.03 100 350.0 10 1.5 × 10⁵ failure 73 Ex. 5

[0115] While specific embodiments of the invention have been describedin detail, it will be appreciated by those skilled in the art thatvarious modifications and alternatives to those details could bedeveloped in light of the overall teachings of the disclosure. Thepresently preferred embodiments described herein are meant to beillustrative only and not limiting as to the scope of the inventionwhich is to be given the full breadth of the appended claims and any andall equivalents thereof.

The invention claimed is:
 1. A pressure sensitive adhesive sheetcomprising a substrate, an intermediate layer superimposed thereon and apressure sensitive adhesive layer superimposed on the intermediatelayer, said pressure sensitive adhesive layer exhibiting an elasticmodulus at 23° C. ranging from 5.0×10⁴ to 1.0×10⁷ Pa, said intermediatelayer exhibiting an elastic modulus at 23° C. which is not greater thanthe elastic modulus at 23° C. of the pressure sensitive adhesive layer.2. The pressure sensitive adhesive sheet as claimed in claim 1, whereinthe substrate exhibits a maximum value of tanδ of dynamicviscoelasticity of at least 0.5 at a temperature ranging from −5 to 80°C.
 3. The pressure sensitive adhesive sheet as claimed in claim 1,wherein the substrate has a thickness and Young's modulus whosemultiplication product is in the range of 0.5 to 100 kg/cm.
 4. Thepressure sensitive adhesive sheet as claimed in claim 2, wherein thesubstrate has a thickness and Young's modulus whose multiplicationproduct is in the range of 0.5 to 100 kg/cm.